The Andromeda galaxy is the best-known galaxy beyond our own, and for good reason: It’s just two-and-a-half million light-years away — the closest giant spiral galaxy.

Recent observations with Hubble Space Telescope have revealed that Andromeda owes a tiny part of its beauty to an encounter with another galaxy.

Astronomers examined the outskirts of Andromeda, 85,000 light-years from its center. The observations indicate that many new stars were born in that region about 1.5 billion to 3 billion years ago.

The reason for Andromeda’s starburst isn’t hard to find. A spiral galaxy in a neighboring constellation is likely responsible. M33 is smaller than Andromeda, but still a substantial galaxy. In fact, it’s the third-largest galaxy in the Local Group, the gathering of several dozen galaxies that includes our own. Only Andromeda and the Milky Way are larger.

About three billion years ago, M33 swung by Andromeda and stirred up the other galaxy’s gas, which then collapsed to form new stars. Andromeda’s gravity did the same to M33, whose outskirts have stars of a similar age, so each galaxy likely enhanced the beauty of the other.

Both galaxies are in the sky tonight. Under dark skies, Andromeda is visible to the unaided eye as a faint smudge of light, about a third of the way up the east-northeastern sky at nightfall. M33 is well below it, and it’s fainter, so you need binoculars or a telescope to pick it out.

Script by Ken Croswell, Copyright 2012

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